This application discloses a new, improved class of satellite constellation that provides continuous satellite coverage of the earth's surface above 25.degree. of north latitude, and by extension global coverage in proportion to the population densities by latitude. The constellation consists of satellites in low altitude (below 4,000 kilometers) inclined elliptical orbits with northern apogees, and in its extension with a subconstellation of satellites with equatorial orbits, these being either circular or elliptical.
The basic constellation provides maximum satellite coverage above 25.degree. of north latitude, that is, over the earth's northern temperate zones. It is optimized to provide service to the United States, Canada, Europe, the Middle East, the former Soviet Union, China, Korea and Japan. In its extension the constellation provides maximum coverage and satellite resources above 25.degree. of north latitude, where the bulk of the world's population lies; it further provides gradually reduced satellite resources but continuous coverage below 25.degree. north, down to 50.degree. of south latitude with minimum horizon look angles of 15.degree. at all times.
The amount of coverage and deployed satellite resources can be increased by overlaying arrays of satellites with suitable offsets or by varying the number of satellites in each subconstellation. The invention applies to any constellation of satellites of the earth where at least twelve satellites in the constellation are in inclined elliptical orbits with apogees in the northern hemisphere below 4,000 kilometers in altitude, and at least six satellites in the constellation are in one equatorial orbit.
While communications satellites, by virtue of their altitudes which can range from hundreds to thousands of kilometers above the earth, can see and serve large areas of the earth at once, satellites are very limited in how they move about the earth by the well known laws of planetary motion. For example, all satellites must revolve about the earth following an elliptical path (including circular paths, which are ellipses of zero eccentricity), and their orbits always lie in a plane that contains the center of the earth. Those seeking to provide satellite communications services have for many years sought ways to construct orbits or constellations of orbits that, while obeying the laws of planetary motion, tailored the amount of satellite coverage and of satellite resources made available to conform to the demand of services in specific geographic areas.
Early satellite orbits were relatively circular and low in altitude (only several hundred kilometers). In such low orbits, the satellites rotate around the earth at speeds higher than earth's own rotation about its axis; consequently a single satellite is in view of a specific region of earth for only a limited time interval. Many satellites are thus required to provide continuous coverage to a particular service area.
It was quickly realized that the inclination, relative to the equator, of the orbital plane of these satellites could be adjusted to optimize the coverage that the satellite constellation provided to the preferred service areas. The Echo satellites were an example of such an approach. This approach tailored the maximum extent in latitude north and south to which satellites would provide service. However coverage was symmetrical about the equator, with an unavoidable fixed distribution of coverage and other satellite resources having a slight preference in coverage for latitudes near the latitude limits over those near the equator.
Later, equatorial geosynchronous satellites became feasible and the preferred approach to provide optimum satellite coverage to a specific service area. Satellites launched into this circular orbit, having a period of 24 hours and zero inclination, revolve about the earth in step with the rotation of the earth. Consequently the satellite appears to remain stationary above a fixed point on the earth's surface. This orbit permits adjusting the satellite longitudinal coverage on the earth to a preferred longitude, but the satellite latitudinal coverage is fixed in a pattern that is symmetrical about the equator, and that favors areas nearest the equator. Moreover, satellites in these orbits do not provide coverage beyond about 70.degree. latitude north or south. These orbits are also very high (22,300 miles), resulting in more expensive satellite and launch costs. Signal delays to and from these satellites can be objectionable, and signal path losses are high, requiring more powerful and sensitive communications equipment.
In recent times another difficulty with geosynchronous satellites has arisen. The geosynchronous equatorial orbit used by these satellites is becoming very crowded. Available positions for future satellites--orbital "slots"--are dwindling.
In order to compensate for poor coverage at high latitudes, as provided by geosynchronous satellites, orbits inclined relative to the equator were considered. For example the United States Milstar satellite system was designed to use inclined circular orbits to obtain better coverage at higher latitudes. However, even in this case, the satellites spend as much time over the southern hemisphere as over the northern.
To provide better and selective coverage at high latitudes, the Soviet Union developed the Molniya satellite system using highly elliptical orbits with apogees in the northern hemisphere. This constellation provides better coverage at higher northern latitudes. The relatively high orbit, (about 40,000 km of apogee by 426 km of perigee with a 12 hour orbital period), combined with the high inclination (63.4.degree.), of the Molniya satellites gives some longitude preference to the Soviet Union and the United States over other longitudes. This inclination prevents precession of the line of apsides (line containing the semi-major axis of the ellipse) around the orbit with an argument of perigee of 270.degree., i.e. apogee at the northernmost point of the orbit. Since in inclined elliptical orbits satellites appear to linger at the apogee and race past the point of perigee, these orbits provide very good coverage of the northern latitudes of the former Soviet Union while spending relatively little time in the southern latitudes where they are not needed. Molniya satellites in orbits with a northern apogee in fact do not, and are not intended to, provide continuous coverage of southern latitudes. The Washington/Moscow Hot line used Molniya satellites.
A combination of Molniya orbits, some with northern apogees and some with southern apogees, could provide global coverage. However, coverage would be symmetrical about the equator.
U.S. Pat. No. 4,854,527 describes a constellation of elliptical orbits that provides continuous coverage at all altitudes (above those which correspond to a 27-hour period) with a minimum of four satellites, two having apogees in the northern hemisphere and two in the southern hemisphere. The satellites define the points of a continually rotating tetrahedron whose sides never or almost never intersect the earth.
Several observations can be made concerning the Draim constellation. First, continuous coverage using this constellation requires satellite periods of at least 27 hours and concomitant altitudes even higher than those of geosynchronous altitudes. As a result, satellites for these orbits would be expensive to build and launch, and voice communications would suffer from unacceptable delays. Second, minimum elevation angles are very low, 4.degree. or less, even for constellations with periods of as much as 100 hours. For many satellite systems, and especially so in mobile applications, such low elevation angles would be impractical. And third, while this constellation provides continuous global coverage, it shows relatively little preference for one region over another. In fact, the very high satellite operating altitudes would mitigate against significant regional preferences. By requiring the existence of at least a tetrahedron, or a polyhedron by extension, the Draim constellation does not offer the possibility to serve selectively one hemisphere. Nor does Draim allow us to tailor the satellite resources offered in one area to existing conditions, such as population density.
Draim is basically and practically symmetrical above and below the equator: coverage in the southern latitudes would be as intense as that in the northern latitudes, despite the fact that there is much less land mass in the southern latitudes than in the northern latitudes. Coverage of the equator is also weaker than in the southern latitudes, just the opposite of that which is desired. Even for asymmetrical subconstellations using the Draim basic configuration (the tetrahedron), when the northern elliptical subconstellation would have different orbital parameters than the southern subconstellation, the two subsets are practically independent of each other. In fact resources deployed for southern use do not contribute to northern capabilities.
The Molniya and Draim satellites have very high apogees, essentially the same as the altitude of geosynchronous satellites, thus they share the same delay problems and suffer from the corresponding complexity and cost constraints associated with satellites operating at such high altitudes.
Lower altitude elliptical orbits can reduce the expense and complexity of the satellites, but at the expense of coverage at lower latitudes and a requirement for more satellites to provide continuous coverage. The earth's Van Allen radiation belt prevents the use of intermediate altitudes that transit the belts (between around 3,000 and 8,000 kilometers).
The ELLIPSO I satellite system, filed before the United States Federal Communications Commission in November, 1990, FCC file number 11-DSS-P-91(6), "Ellipso I, An Elliptical Orbit Satellite System", and Public Notice, DA91-407, released Apr. 1, 1991 also used elliptical orbits with northern apogees to provide preferred coverage to the temperate zone of the northern hemisphere. This zone contains the bulk of the likely market in the near future for mobile satellite communication services, for which the ELLIPSO I system was intended. This satellite constellation uses lower orbital altitudes ("LEO") in order to lessen satellite costs and provide service at all northern latitudes around the globe, regardless of longitude. The ELLIPSO I satellite orbits have maximum altitudes of 1250 kilometers and minimum altitudes of 500 kilometers, inclined at 63.4.degree.. Like the Molniya satellite system, the ELLIPSO I system did not provide continuous coverage below a specified altitude, in ELLIPSO I's case, below around 30.degree. north latitude.
Other circular LEO systems of the same class have subsequently been proposed involving a much higher number of satellites to provide homogeneous service, uniformly distributed around the earth. For example, Motorola's Iridium system consists of 77 satellites in circular polar orbit. Another example is Loral Systems Globalstar, an inclined circular LEO constellation of 48 satellites. But a look at a globe of the earth will reveal that there is much more populated land mass north of the equator than south of it. In addition, the distribution of land mass is not at all symmetric about the equator. Therefore circular orbit LEO constellations are not efficient in providing coverage in proportion to the earth's populated land masses.
In conclusion, notwithstanding the approaches taken by the above described systems, no low altitude satellite constellation has yet been developed that is optimized to cover all the areas of the northern temperate latitudes in a proper proportion. Furthermore, no constellation has been developed, revealed, or proposed, that is tailored to provide coverage to the populated land masses of the earth as a whole, and wherein such coverage is in proportion to the population densities of said land masses. Finally no constellation has been proposed that permits its gradual deployment, while demand develops and partially financed by said demand; this unique aspect of the invention herein offers a most important, and perhaps essential economic basis for the deployment of the constellation.
Satellite constellations to date have required the entire capital investment to be spent before services are offered and revenues generated; this prevents the financing of the system, at least partially, to be made from revenues generated from usage fees. In addition to other aims related to the optimization of coverage and cost of the constellation, this invention aims also at emulating for the first time for satellite systems the economic environment in which terrestrial system are deployed, gradual and in step with the development of user demand. The presentation and claim of such a class of constellation is the object of this invention. The principal characteristics of the present invention were publicly disclosed in a filing with the Federal Communications Commission submitted on Jun. 3, 1991: FCC File no. 18-DDS-P-91(18), "ELLIPSO II, An Elliptical Orbit Satellite System." See also Public Notice DA 91-1308 released Oct. 14, 1991.
In consequence of the above observations, the objects of the present invention are as follows.
Provision of continuous satellite coverage of the earth's northern temperate zone from low (below 4000 km) satellite altitudes.
Provision of continuous satellite coverage of the earth's surface that is in proportion to the distribution of population over the earth by latitude. In distinction, continuous global coverage of the earth is not an required objective of the invention.
Provision of a constellation of satellites that permits the gradual and progressive deployment to conform to local demand requirements.
Provision of a constellation of satellites that makes efficient use of satellite resources for continuously serving populations of users around the earth in the northern temperate zone while minimizing coverage of other areas.
Provision of a constellation of satellites that makes efficient use of satellite resources while providing coverage to regions of the earth in proportion to population density, without wasting satellite resources in areas of low population density, such as the southern oceans or the polar regions.
Provision of satellite service to mobile and fixed users, using satellites in orbits that are relatively low compared to satellites in geosynchronous equatorial orbit, so as to reduce satellite launch expense, satellite complexity and weight, and communications path delay times.
Provision of continuous satellite coverage over the earth's northern temperate zone in such a way as to obtain the most service for the least number of satellites, all else being equal.
Provision of continuous satellite coverage over the world's populated land masses in such a way as to obtain the most service, mobile and fixed, for the least number of satellites, all else being equal.
Provision of continuous satellite service to the world's populated areas at inexpensive service rates especially for mobile service as a consequence of an efficient constellation design.
Provision of service from a constellation of satellites that is in constant motion relative to any point on earth, so that if any satellite failure occurs within the constellation, no one service region will be catastrophically penalized above any other. Instead a much smaller reduction in service will occur over a larger area.